arXiv subjects

Jakob S. den Brok

Publications and source records attributed to Jakob S. den Brok.

18 recordsLinked to original sources

arXiv2025

BASS.L. Near-infrared Data Release 3: A Spectral Atlas and Characterization of AGN

We present an analysis of near-infrared (NIR) emission-line properties, AGN diagnostics, and circumnuclear gas dynamics for 453 hard X-ray selected (14$-$195 keV) AGN from the BAT AGN Spectroscopic Survey (BASS) NIR Data Release 3 (DR3; $\langle z \rangle=0.036$, $z<1.0$). This dataset is the largest compilation of rest-frame NIR spectroscopic observations of hard-X-ray-selected AGN and includes the full DR2 sample. Observations were obtained with VLT X-shooter, a multiwavelength (0.3$-$2.5$μ$m) spectrograph ($R$ = 4,000$-$18,000), using a $\geq 2 σ$ detection threshold, enabling broad analysis of emission features. We find that NIR coronal lines, particularly SiVI $\lambda1.964$, are more reliable tracers of AGN luminosity than optical [OIII], showing a tighter correlation with hard X-ray luminosity ($σ=0.25$dex) than [OIII] $\lambda5007$ ($σ=0.55$dex). Broad Paschen lines (Pa$α$ and Pa$β$) are detected in 12% of Seyfert 2 and 57% of Seyfert 1.9, consistent with previous hidden BLR studies. We introduce a refined NIR diagnostic diagram (FeII $\lambda1.257$$μ$m/Pa$β$ and H$_2$ $λ2.122$$μ$m/Br$γ$) that effectively distinguishes AGN, star-forming, and composite sources even when contamination limits individual diagnostics or only upper limits are available. Additionally, we find a moderate correlation ($p \approx 7.4 \times 10^{-3}$) between hot molecular gas mass (traced by H$_2$ 2.121$μ$m) and X-ray luminosity, while its relation with Eddington ratio is weaker. The hot-to-cold gas mass ratio spans four orders of magnitude, averaging $\sim 3 \times 10^{-7}$, indicating diverse molecular gas excitation processes likely driven by star formation and AGN feedback. Our results underscore the value of NIR spectroscopy in probing AGN activity, obscured BLRs, and the complex interactions between AGN and their circumnuclear environments.

astro-ph.GA↗

arXiv2024

The properties and kinematics of HCN emission across the closest starburst galaxy NGC 253 observed with ALMA

Studying molecular gas in nearby galaxies using hydrogen cyanide (HCN) as a tracer for higher densities than CO emission still poses a significant challenge. Even though several galaxies have HCN maps on a few kpc scales, higher-resolution maps are still required. Our goal is to examine the contrast in intensity between two tracers that probe different density regimes - HCN(1-0)/CO(2-1) ratio - and their kinematics across NGC 253. By utilizing the advanced capabilities of the Atacama Large Millimeter/submillimeter Array (ALMA), we can map these features at high resolution across a large field of view and uncover the nature of such dense gas in extragalactic systems. We present new ALMA Atacama Compact Array and Total Power (ACA+TP) observations of the HCN emission across NGC 253, covering the inner 8.6' of the galaxy disk at 300 pc scales. We analyze the integrated intensity and mean velocity of HCN and CO along each line of sight and use SCOUSE software to perform spectral decomposition, which considers each velocity component separately. Molecular gas traced by HCN piles up in a ring-like structure at a radius of 2 kpc. The HCN emission is enhanced by 2 orders of magnitude in the central 2 kpc regions, beyond which its intensity decreases with increasing galactocentric distance. The number of components in the HCN spectra shows a robust environmental dependence, with multiple velocity features across the center and bar. We have identified an increase in the HCN/CO ratio in these regions, corresponding to a velocity component likely associated with a molecular outflow. We have also discovered that the ratio between the total infrared luminosity and dense gas mass, which indicates the star formation efficiency of dense gas, is anti-correlated with the molecular gas surface density up to approximately 200 Msul/pc^2. In contrast, beyond this point, the ratio starts to increase.

astro-ph.GA↗

arXiv2023

BASS XXXIV: A Catalog of the Nuclear Mm-wave Continuum Emission Properties of AGNs Constrained on Scales $\lesssim$ 100--200 pc

We present a catalog of the millimeter-wave (mm-wave) continuum properties of 98 nearby ($z <$ 0.05) active galactic nuclei (AGNs) selected from the 70-month Swift/BAT hard X-ray catalog that have precisely determined X-ray spectral properties and subarcsec-resolution ALMA Band-6 (211--275 GHz) observations as of 2021 April. Due to the hard-X-ray ($>$ 10 keV) selection, the sample is nearly unbiased for obscured systems at least up to Compton-thick-level obscuration, and provides the largest number of AGNs with high physical resolution mm-wave data ($\lesssim$ 100--200 pc). Our catalog reports emission peak coordinates, spectral indices, and peak fluxes and luminosities at 1.3 mm (230 GHz). Additionally, high-resolution mm-wave images are provided. Using the images and creating radial surface brightness profiles of mm-wave emission, we identify emission extending from the central source and isolated blob-like emission. Flags indicating the presence of these emission features are tabulated. Among 90 AGNs with significant detections of nuclear emission, 37 AGNs ($\approx$ 41%) appear to have both or one of extended or blob-like components. We, in particular, investigate AGNs that show well-resolved mm-wave components and find that these seem to have a variety of origins (i.e., a jet, radio lobes, a secondary AGN, stellar clusters, a narrow line region, galaxy disk, active star-formation regions, and AGN-driven outflows), and some components have currently unclear origins.

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arXiv2023

Spectral Stacking of Radio-Interferometric Data

Mapping molecular line emission beyond the bright low-J CO transitions is still challenging in extragalactic studies, even with the latest generation of (sub-)mm interferometers, such as ALMA and NOEMA. We summarise and test a spectral stacking method that has been used in the literature to recover low-intensity molecular line emission, such as HCN(1-0), HCO+(1-0), and even fainter lines in external galaxies. The goal is to study the capabilities and limitations of the stacking technique when applied to imaged interferometric observations. The core idea of spectral stacking is to align spectra of the low S/N spectral lines to a known velocity field calculated from a higher S/N line expected to share the kinematics of the fainter line, e.g., CO(1-0) or 21-cm emission. Then these aligned spectra can be coherently averaged to produce potentially high S/N spectral stacks. Here, we use imaged simulated interferometric and total power observations at different signal-to-noise levels, based on real CO observations. For the combined interferometric and total power data, we find that the spectral stacking technique is capable of recovering the integrated intensities even at low S/N levels across most of the region where the high S/N prior is detected. However, when stacking interferometer-only data for low S/N emission, the stacks can miss up to 50% of the emission from the fainter line. A key result of this analysis is that the spectral stacking method is able to recover the true mean line intensities in low S/N cubes and to accurately measure the statistical significance of the recovered lines. To facilitate the application of this technique we provide a public Python package, called PyStacker.

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arXiv2023

Wide-field CO isotopologue emission and the CO-to-H$_2$ factor across the nearby spiral galaxy M101

Carbon monoxide (CO) emission is the most widely used tracer of the bulk molecular gas in the interstellar medium (ISM) in extragalactic studies. The CO-to-H$_2$ conversion factor, $α_{\rm CO}$, links the observed CO emission to the total molecular gas mass. However, no single prescription perfectly describes the variation of $α_{\rm CO}$ across all environments across galaxies as a function of metallicity, molecular gas opacity, line excitation, and other factors. Using resolved spectral line observations of CO and its isotopologues, we can constrain the molecular gas conditions and link them to a variation in the conversion factor. We present new IRAM 30-m 1mm and 3mm line observations of $^{12}$CO, $^{13}$CO, and C$^{18}$O} across the nearby galaxy M101. Based on the CO isotopologue line ratios, we find that selective nucleosynthesis and opacity changes are the main drivers of the variation in the line emission across the galaxy. Furthermore, we estimated $α_{\rm CO(1-0)}$ using different approaches, including (i) the dust mass surface density derived from far-IR emission as an independent tracer of the total gas surface density and (ii) LTE-based measurements using the optically thin $^{13}$CO(1-0) intensity. We find an average value of $α_{\rm CO}=4.4{\pm}0.9\rm\,M_\odot\,pc^{-2}(K\,km\,s^{-1})^{-1}$ across the galaxy, with a decrease by a factor of 10 toward the 2 kpc central region. In contrast, we find LTE-based values are lower by a factor of 2-3 across the disk relative to the dust-based result. Accounting for $α_{\rm CO}$ variations, we found significantly reduced molecular gas depletion time by a factor 10 in the galaxy's center. In conclusion, our result suggests implications for commonly derived scaling relations, such as an underestimation of the slope of the Kennicutt Schmidt law, if $α_{\rm CO}$ variations are not accounted for.

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arXiv2023

The ALMOND Survey: Molecular cloud properties and gas density tracers across 25 nearby spiral galaxies with ALMA

We use new HCN(1-0) data from the ALMOND (ACA Large-sample Mapping Of Nearby galaxies in Dense gas) survey to trace the kpc-scale molecular gas density structure and CO(2-1) data from PHANGS-ALMA to trace the bulk molecular gas across 25 nearby, star-forming galaxies. At 2.1 kpc scale, we measure the density-sensitive HCN/CO line ratio and the SFR/HCN ratio to trace the star formation efficiency in the denser molecular medium. At 150 pc scale, we measure structural and dynamical properties of the molecular gas via CO(2-1) line emission, which is linked to the lower resolution data using an intensity-weighted averaging method. We find positive correlations (negative) of HCN/CO (SFR/HCN) with the surface density, the velocity dispersion and the internal turbulent pressure of the molecular gas. These observed correlations agree with expected trends from turbulent models of star formation, which consider a single free-fall time gravitational collapse. Our results show that the kpc-scale HCN/CO line ratio is a powerful tool to trace the 150 pc scale average density distribution of the molecular clouds. Lastly, we find systematic variations of the SFR/HCN ratio with cloud-scale molecular gas properties, which are incompatible with a universal star formation efficiency. Overall, these findings show that mean molecular gas density, molecular cloud properties and star formation are closely linked in a coherent way, and observations of density-sensitive molecular gas tracers are a useful tool to analyse these variations, linking molecular gas physics to stellar output across galaxy discs.

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arXiv2023

Sub-kiloparsec empirical relations and excitation conditions of HCN and HCO+ J=3-2 in nearby star-forming galaxies

We present new HCN and HCO$^+$ ($J$=3-2) images of the nearby star-forming galaxies (SFGs) NGC 3351, NGC 3627, and NGC 4321. The observations, obtained with the Morita ALMA Compact Array, have a spatial resolution of $\sim$290-440 pc and resolve the inner $R_\textrm{gal} \lesssim$ 0.6-1 kpc of the targets, as well as the southern bar end of NGC 3627. We complement this data set with publicly available images of lower excitation lines of HCN, HCO$^+$, and CO and analyse the behaviour of a representative set of line ratios: HCN(3-2)/HCN(1-0), HCN(3-2)/HCO$^+$(3-2), HCN(1-0)/CO(2-1), and HCN(3-2)/CO(2-1). Most of these ratios peak at the galaxy centres and decrease outwards. We compare the HCN and HCO$^+$ observations with a grid of one-phase, non-local thermodynamic equilibrium (non-LTE) radiative transfer models and find them compatible with models that predict subthermally excited and optically thick lines. We study the systematic variations of the line ratios across the targets as a function of the stellar surface density ($Σ_\textrm{star}$), the intensity-weighted CO(2-1) ($\langle I_\text{CO}\rangle$), and the star formation rate surface density ($Σ_\text{SFR}$). We find no apparent correlation with $Σ_\text{SFR}$, but positive correlations with the other two parameters, which are stronger in the case of $\langle I_\text{CO}\rangle$. The HCN/CO-$\langle I_\text{CO}\rangle$ relations show $\lesssim$0.3 dex galaxy-to-galaxy offsets, with HCN(3-2)/CO(2-1)-$\langle I_\text{CO}\rangle$ being $\sim$2 times steeper than HCN(1-0)/CO(2-1). In contrast, the HCN(3-2)/HCN(1-0)-$\langle I_\text{CO}\rangle$ relation exhibits a tighter alignment between galaxies. We conclude that the overall behaviour of the line ratios cannot be ascribed to variations in a single excitation parameter (e.g. density or temperature).

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arXiv2022

PHANGS-JWST First Results: A Global and Moderately Resolved View of Mid-Infrared and CO Line Emission from Galaxies at the Start of the JWST Era

We explore the relationship between mid-infrared (mid-IR) and CO rotational line emission from massive star-forming galaxies, which is one of the tightest scalings in the local universe. We assemble a large set of unresolved and moderately ($\sim 1$ kpc) spatially resolved measurements of CO (1-0) and CO (2-1) intensity, $I_{\rm CO}$, and mid-IR intensity, $I_{\rm MIR}$, at 8, 12, 22, and 24$μ$m. The $I_{\rm CO}$ vs. $I_{\rm MIR}$ relationship is reasonably described by a power law with slopes $0.7{-}1.2$ and normalization $I_{\rm CO} \sim 1$ K km s$^{-1}$ at $I_{\rm MIR} \sim 1$ MJy sr$^{-1}$. Both the slopes and intercepts vary systematically with choice of line and band. The comparison between the relations measured for CO~(1-0) and CO (2-1) allow us to infer that $R_{21} \propto I_{\rm MIR}^{0.2}$, in good agreement with other work. The $8μ$m and $12μ$m bands, with strong PAH features, show steeper CO vs. mid-IR slopes than the $22μ$m and $24μ$m, consistent with PAH emission arising not just from CO-bright gas but also from atomic or CO-dark gas. The CO-to-mid-IR ratio correlates with global galaxy stellar mass ($M_\star$) and anti-correlates with SFR/$M_\star$. At $\sim 1$ kpc resolution, the first four PHANGS-JWST targets show CO to mid-IR relationships that are quantitatively similar to our larger literature sample, including showing the steep CO-to-mid-IR slopes for the JWST PAH-tracing bands, although we caution that these initial data have a small sample size and span a limited range of intensities.

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arXiv2022

PHANGS-JWST First Results: Stellar Feedback-Driven Excitation and Dissociation of Molecular Gas in the Starburst Ring of NGC 1365?

We compare embedded young massive star clusters (YMCs) to (sub-)millimeter line observations tracing the excitation and dissociation of molecular gas in the starburst ring of NGC 1365. This galaxy hosts one of the strongest nuclear starbursts and richest populations of YMCs within 20 Mpc. Here we combine near-/mid-IR PHANGS-JWST imaging with new ALMA multi-J CO (1-0, 2-1 and 4-3) and [CI](1-0) mapping, which we use to trace CO excitation via R42 = I_CO(4-3)/I_CO(2-1) and R21 = I_CO(2-1)/I_CO(1-0) and dissociation via RCICO = I_[CI](1-0)/I_CO(2-1) at 330 pc resolution. We find that the gas flowing into the starburst ring from northeast to southwest appears strongly affected by stellar feedback, showing decreased excitation (lower R42) and increased signatures of dissociation (higher RCICO) in the downstream regions. There, radiative transfer modeling suggests that the molecular gas density decreases and temperature and [CI/CO] abundance ratio increase. We compare R42 and RCICO with local conditions across the regions and find that both correlate with near-IR 2 um emission tracing the YMCs and with both PAH (11.3 um) and dust continuum (21 um) emission. In general, RCICO exhibits ~ 0.1 dex tighter correlations than R42, suggesting CI to be a more sensitive tracer of changing physical conditions in the NGC 1365 starburst than CO (4-3). Our results are consistent with a scenario where gas flows into the two arm regions along the bar, becomes condensed/shocked, forms YMCs, and then these YMCs heat and dissociate the gas.

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arXiv2022

PHANGS-JWST First Results: Multi-wavelength view of feedback-driven bubbles (The Phantom Voids) across NGC 628

We present a high-resolution view of bubbles within The Phantom Galaxy (NGC 628); a nearby (~10Mpc), star-forming (~2Msun/yr), face-on (i~9deg) grand-design spiral galaxy. With new data obtained as part of the PHANGS-JWST treasury program, we perform a detailed case-study of two regions of interest, one of which contains the largest and most prominent bubble in the galaxy (The Phantom Void; over 1kpc in diameter), and the other being a smaller region that may be the precursor to such a large bubble (The Precursor Phantom Void). When comparing to matched resolution Halpha observations from the Hubble Space Telescope (HST), we see that the ionized gas is brightest in the shells of both bubbles, and is coincident with the youngest (~1Myr) and most massive (~100,000Msun) stellar associations. We also find an older generation (~20Myr) of stellar associations is present within the bubble of The Phantom Void. From our kinematic analysis of the HI, H2 (CO) and HII gas across The Phantom Void, we infer a high expansion speed of around 15 to 50km/s. The large size and high expansion speed of The Phantom Void suggest that the driving mechanism is sustained stellar feedback due to multiple mechanisms, where early feedback first cleared a bubble (as we observe now in The Precursor Phantom Void), and since then SNe have been exploding within the cavity, and have accelerated the shell. Finally, comparison to simulations shows a striking resemblance to our JWST observations, and suggests that such large-scale stellar feedback-driven bubbles should be common within other galaxies.

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arXiv2022

BASS XXXII: Studying the Nuclear Mm-wave Continuum Emission of AGNs with ALMA at Scales $\lesssim$ 100-200 pc

To understand the origin of nuclear ($\lesssim$ 100 pc) millimeter-wave (mm-wave) continuum emission in active galactic nuclei (AGNs), we systematically analyzed sub-arcsec resolution Band-6 (211-275 GHz) ALMA data of 98 nearby AGNs ($z <$ 0.05) from the 70-month Swift/BAT catalog. The sample, almost unbiased for obscured systems, provides the largest number of AGNs to date with high mm-wave spatial resolution sampling ($\sim$ 1-200 pc), and spans broad ranges of 14-150 keV luminosity {$40 < \log[L_{\rm 14-150}/({\rm erg\,s^{-1}})] < 45$}, black hole mass [$5 < \log(M_{\rm BH}/M_\odot) < 10$], and Eddington ratio ($-4 < \log λ_{\rm Edd} < 2$). We find a significant correlation between 1.3 mm (230 GHz) and 14-150 keV luminosities. Its scatter is $\approx$ 0.36 dex, and the mm-wave emission may serve as a good proxy of the AGN luminosity, free of dust extinction up to $N_{\rm H} \sim 10^{26}$ cm$^{-2}$. While the mm-wave emission could be self-absorbed synchrotron radiation around the X-ray corona according to past works, we also discuss different possible origins of the mm-wave emission; AGN-related dust emission, outflow-driven shocks, and a small-scale ($<$ 200 pc) jet. The dust emission is unlikely to be dominant, as the mm-wave slope is generally flatter than expected. Also, due to no increase in the mm-wave luminosity with the Eddington ratio, a radiation-driven outflow model is possibly not the common mechanism. Furthermore, we find independence of the mm-wave luminosity on indicators of the inclination angle from the polar axis of the nuclear structure, which is inconsistent with a jet model whose luminosity depends only on the angle.

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arXiv2022

BASS XXVI: DR2 Host Galaxy Stellar Velocity Dispersions

We present new central stellar velocity dispersions for 484 Sy 1.9 and Sy 2 from the second data release of the Swift/BAT AGN Spectroscopic Survey (BASS DR2). This constitutes the largest study of velocity dispersion measurements in X-ray selected, obscured AGN with 956 independent measurements of the Ca H+K and Mg b region (3880-5550A) and the Ca triplet region (8350-8730A) from 642 spectra mainly from VLT/Xshooter or Palomar/DoubleSpec. Our sample spans velocity dispersions of 40-360 km/s, corresponding to 4-5 orders of magnitude in black holes mass (MBH=10^5.5-9.6 Msun), bolometric luminosity (LBol~10^{42-46 ergs/s), and Eddington ratio (L/Ledd~10^{-5}-2). For 281 AGN, our data provide the first published central velocity dispersions, including 6 AGN with low mass black holes (MBH=10^5.5-6.5 Msun), discovered thanks to our high spectral resolution observations (sigma~25 km/s). The survey represents a significant advance with a nearly complete census of hard-X-ray selected obscured AGN with measurements for 99% of nearby AGN (z<0.1) outside the Galactic plane. The BASS AGN have higher velocity dispersions than the more numerous optically selected narrow line AGN (i.e., ~150 vs. ~100 km/s), but are not biased towards the highest velocity dispersions of massive ellipticals (i.e., >250 km/s). Despite sufficient spectral resolution to resolve the velocity dispersions associated with the bulges of small black holes (~10^4-5 Msun), we do not find a significant population of super-Eddington AGN. Using estimates of the black hole sphere of influence, direct stellar and gas black hole mass measurements could be obtained with existing facilities for more than ~100 BASS AGN.

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arXiv2022

BASS XXII: The BASS DR2 AGN Catalog and Data

We present the AGN catalog and optical spectroscopy for the second data release of the Swift BAT AGN Spectroscopic Survey (BASS DR2). With this DR2 release we provide 1425 optical spectra, of which 1181 are released for the first time, for the 858 hard X-ray selected AGN in the Swift BAT 70-month sample. The majority of the spectra (813/1425, 57%) are newly obtained from VLT/Xshooter or Palomar/Doublespec. Many of the spectra have both higher resolution (R>2500, N~450) and/or very wide wavelength coverage (3200-10000 A, N~600) that are important for a variety of AGN and host galaxy studies. We include newly revised AGN counterparts for the full sample and review important issues for population studies, with 44 AGN redshifts determined for the first time and 780 black hole mass and accretion rate estimates. This release is spectroscopically complete for all AGN (100%, 858/858) with 99.8% having redshift measurements (857/858) and 96% completion in black hole mass estimates of unbeamed AGN (outside the Galactic plane). This AGN sample represents a unique census of the brightest hard X-ray selected AGN in the sky, spanning many orders of magnitude in Eddington ratio (Ledd=10^-5-100), black hole mass (MBH=10^5-10^10 Msun), and AGN bolometric luminosity (Lbol=10^40-10^47 ergs/s).

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arXiv2022

BAT AGN Spectroscopic Survey XXI: The Data Release 2 Overview

The BAT AGN Spectroscopic Survey (BASS) is designed to provide a highly complete census of the key physical parameters of supermassive black holes (SMBHs) that power local active galactic nuclei (AGN) (z<0.3), including their bolometric luminosity, black hole mass, accretion rates, and line-of-sight gas obscuration, and the distinctive properties of their host galaxies (e.g., star formation rates, masses, and gas fractions). We present an overview of the BASS data release 2 (DR2), an unprecedented spectroscopic survey in spectral range, resolution, and sensitivity, including 1449 optical (3200-10000 A) and 233 NIR (1-2.5 um) spectra for the brightest 858 ultra-hard X-ray (14-195 keV) selected AGN across the entire sky and essentially all levels of obscuration. This release provides a highly complete set of key measurements (emission line measurements and central velocity dispersions), with 99.9% measured redshifts and 98% black hole masses estimated (for unbeamed AGN outside the Galactic plane). The BASS DR2 AGN sample represents a unique census of nearby powerful AGN, spanning over 5 orders of magnitude in AGN bolometric luminosity, black hole mass, Eddington ratio, and obscuration. The public BASS DR2 sample and measurements can thus be used to answer fundamental questions about SMBH growth and its links to host galaxy evolution and feedback in the local universe, as well as open questions concerning SMBH physics. Here we provide a brief overview of the survey strategy, the key BASS DR2 measurements, data sets and catalogs, and scientific highlights from a series of DR2-based works.

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arXiv2022

A CO isotopologue Line Atlas within the Whirlpool galaxy Survey (CLAWS)

We present the CO isotopologue Line Atlas within the Whirpool galaxy Survey (CLAWS) based on an IRAM 30-m large programme which provides a benchmark study of numerous, faint CO isotopologues in the mm-wavelength regime across the full disc of M51 (NGC 5194). The survey's core goal is to use the low-J CO isotopologue lines to constrain CO excitation and chemistry, and therefrom the local physical conditions of the gas. In this survey paper, we describe the CLAWS observing and data reduction strategies. We map the J=1-0 and 2-1 transitions of the CO isotopologues $^{12}$CO,$^{13}$CO, C$^{18}$O and C$^{17}$O, as well as several supplementary lines within the 1 mm and 3 mm window (CN(1-0), CS(2-1), CH$_3$OH(2-1), N$_2$H$^+$(1-0), HC$_3$N(10-9)) at ~1 kpc resolution. A total observation time of 149 h offers unprecedented sensitivity. We use these data to explore several CO isotopologue line ratios in detail, study their radial (and azimuthal) trends and investigate whether changes in line ratios stem from changes in ISM properties such as gas temperatures, densities or chemical abundances. For example, we find negative radial trends for the $^{13}$CO}/$^{12}$CO, C$^{18}$O/$^{12}$CO and C$^{18}$O/$^{13}$CO line ratios in their J=1-0 transitions. We also find variations with local environment, such as higher $^{12}$CO(2-1)/(1-0) or $^{13}$CO/$^{12}$CO(1-0) line ratios in interarm regions compared to spiral arm regions. We propose that these aforementioned variations of CO line ratios are most likely due to a variation of the optical depth, while abundance variations due to selective nucleosynthesis on a galaxy-wide scale could also play a role. We also study the CO spectral line energy distribution (SLED) using archival JCMT $^{12}$CO(3-2) data and find a variation of the SLED shape with local environmental parameters further underlying changes in optical depth, gas temperatures or densities.

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arXiv2021

Low-J CO Line Ratios From Single Dish CO Mapping Surveys and PHANGS-ALMA

We measure the low-J CO line ratio R21=CO(2-1)/CO(1-0), R32=CO(3-2)/CO(2-1), and R31 = CO(3-2)/CO(1-0) using whole-disk CO maps of nearby galaxies. We draw CO(2-1) from PHANGS--ALMA, HERACLES, and follow-up IRAM surveys; CO(1-0) from COMING and the Nobeyama CO Atlas of Nearby Spiral Galaxies; and CO(3-2) from the JCMT NGLS and APEX LASMA mapping. Altogether this yields 76, 47, and 29 maps of R21, R32, and R31 at 20" \sim 1.3 kpc resolution, covering 43, 34, and 20 galaxies. Disk galaxies with high stellar mass, log10 M_* [Msun]=10.25-11 and star formation rate, SFR=1-5 Msun/yr, dominate the sample. We find galaxy-integrated mean values and 16%-84% range of R21 = 0.65 (0.50-0.83), R32=0.50 (0.23-0.59), and R31=0.31 (0.20-0.42). We identify weak trends relating galaxy-integrated line ratios to properties expected to correlate with excitation, including SFR/M_* and SFR/L_CO. Within galaxies, we measure central enhancements with respect to the galaxy-averaged value of \sim 0.18^{+0.09}_{-0.14} dex for R21, 0.27^{+0.13}_{-0.15} dex for R31, and 0.08^{+0.11}_{-0.09} dex for R32. All three line ratios anti-correlate with galactocentric radius and positively correlate with the local star formation rate surface density and specific star formation rate, and we provide approximate fits to these relations. The observed ratios can be reasonably reproduced by models with low temperature, moderate opacity, and moderate densities, in good agreement with expectations for the cold ISM. Because the line ratios are expected to anti-correlate with the CO(1-0)-to-H_2 conversion factor, alphaCO^(1-0), these results have general implications for the interpretation of CO emission from galaxies.

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arXiv2021

PHANGS-ALMA: Arcsecond CO(2-1) Imaging of Nearby Star-Forming Galaxies

We present PHANGS-ALMA, the first survey to map CO J=2-1 line emission at ~1" ~ 100pc spatial resolution from a representative sample of 90 nearby (d<~20 Mpc) galaxies that lie on or near the z=0 "main sequence" of star-forming galaxies. CO line emission traces the bulk distribution of molecular gas, which is the cold, star-forming phase of the interstellar medium. At the resolution achieved by PHANGS-ALMA, each beam reaches the size of a typical individual giant molecular cloud (GMC), so that these data can be used to measure the demographics, life-cycle, and physical state of molecular clouds across the population of galaxies where the majority of stars form at z=0. This paper describes the scientific motivation and background for the survey, sample selection, global properties of the targets, ALMA observations, and characteristics of the delivered ALMA data and derived data products. As the ALMA sample serves as the parent sample for parallel surveys with VLT/MUSE, HST, AstroSat, VLA, and other facilities, we include a detailed discussion of the sample selection. We detail the estimation of galaxy mass, size, star formation rate, CO luminosity, and other properties, compare estimates using different systems and provide best-estimate integrated measurements for each target. We also report the design and execution of the ALMA observations, which combine a Cycle~5 Large Program, a series of smaller programs, and archival observations. Finally, we present the first 1" resolution atlas of CO emission from nearby galaxies and describe the properties and contents of the first PHANGS-ALMA public data release.

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arXiv2021

PHANGS-ALMA Data Processing and Pipeline

We describe the processing of the PHANGS-ALMA survey and present the PHANGS-ALMA pipeline, a public software package that processes calibrated interferometric and total power data into science-ready data products. PHANGS-ALMA is a large, high-resolution survey of CO J=2-1 emission from nearby galaxies. The observations combine ALMA's main 12-m array, the 7-m array, and total power observations and use mosaics of dozens to hundreds of individual pointings. We describe the processing of the u-v data, imaging and deconvolution, linear mosaicking, combining interferometer and total power data, noise estimation, masking, data product creation, and quality assurance. Our pipeline has a general design and can also be applied to VLA and ALMA observations of other spectral lines and continuum emission. We highlight our recipe for deconvolution of complex spectral line observations, which combines multiscale clean, single scale clean, and automatic mask generation in a way that appears robust and effective. We also emphasize our two-track approach to masking and data product creation. We construct one set of "broadly masked" data products, which have high completeness but significant contamination by noise, and another set of "strictly masked" data products, which have high confidence but exclude faint, low signal-to-noise emission. Our quality assurance tests, supported by simulations, demonstrate that 12-m+7-m deconvolved data recover a total flux that is significantly closer to the total power flux than the 7-m deconvolved data alone. In the appendices, we measure the stability of the ALMA total power calibration in PHANGS--ALMA and test the performance of popular short-spacing correction algorithms.

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